(function (doc, win) {
    var docEl = doc.documentElement,
         resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
         recalc    = function () {
                var clientWidth = docEl.clientWidth;
                if (clientWidth>=640) {
                   clientWidth = 640;
                };
                if (!clientWidth) return;
              docEl.style.fontSize = 100 * (clientWidth / 640) + 'px';
         };
         if (!doc.addEventListener) return;
         win.addEventListener(resizeEvt, recalc, false);
         doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);